1. Field of the Invention
The present invention relates to a display panel in which a light emitting element formed on a substrate is sealed between the substrate and a cover member. Also, the present invention relates to a display module in which an IC is mounted in the display panel. Note that, in this specification, the display panel and the display module are generically called a light emitting device. Further, the present invention relates to electronic equipment using the light emitting device.
2. Description of the Related Art
Since a light emitting element itself emits light, visibility is high, a back light required for a liquid crystal display device (LCD) is unnecessary, it is suitable for thinness, and there is no limitation for a view angle. Thus, recently, a light emitting device using a light emitting element is noted as a display device alternative to a CRT and an LCD.
The light emitting element has a layer including an organic compound in which luminescence (electroluminescence) is produced by applying an electric field thereto (hereinafter referred to as an organic compound layer), an anode layer, and a cathode layer. The luminescence in the organic compound includes luminescence produced when it is returned from a singlet excitation state to a ground state (fluorescence) and luminescence produced when it is returned from a triplet excitation state to the ground state (phosphorescence). In the light emitting device of the present invention, either luminescence may be employed.
Note that, in this specification, all layers provided between the anode and the cathode are defined as the organic compound layers. The organic compound layer includes specifically a light emitting layer, a hole injection layer, an electron injection layer, a hole transport layer, and an electron transport layer. Basically, the light emitting element has a structure in which the anode, the light emitting layer, and the cathode are laminated in order. In addition to this structure, there are the case where the light emitting element has a structure in which the anode, the hole injection layer, the light emitting layer, and the cathode are laminated in this order and the case where the light emitting element has a structure in which the anode, the hole injection layer, the light emitting layer, the electron transport layer, and the cathode are laminated in this order.
Also, in this specification, light emission by the light emitting element is called operation of the light emitting element. Further, in this specification, an element composed of the anode, the organic compound layer, and the cathode is called the light emitting element.
Recently, the use of an active matrix light emitting device is expanded and the needs for a large size of a screen, high definition, and high reliability are increased. Simultaneously, the improvement of productivity and the need for a low cost are increased.
In the active matrix light emitting device, a current flowing into the light emitting element is controlled by a thin film transistor (TFT) provided in respective pixels.
Conventionally, when the TFT is manufactured using aluminum as a material for a gate signal line in the above-mentioned TFT, a protrusion such as a hillock or a whisker is formed by thermal treatment and an aluminum atom is diffused into a channel forming region. Therefore, a failure in the operation of the TFT and a deterioration of a TFT characteristic are caused. To prevent this, when a metal material resistant to the thermal treatment, typically, a metal element having a high melting point is used, a wiring resistance becomes higher in the case where a screen size is made large. Thus, a problem such as an increase of consumption power is caused. With respect to the light emitting element, a consumption current is large. Therefore, particularly, in the case of a panel with 3 inches or more, due to the influence of the wiring resistance, intensities in both ends of the screen are different from each other or crosstalk is caused.